1. Field of the Invention
The present invention relates to high accuracy diffraction interferometry, and more specifically, it relates to the use of embodiments of the Phase Shifting Interferometer to measure the aberrations of convex lenses and negative lenses.
2. Description of Related Art
Interferometry is the preferred method to measure the performance of optical elements and systems. In this method the wavefront of light reflected from or transmitted by the optic to be tested is interfered with the wavefront from a reference surface, to produce an interference fringe pattern. These interference fringes are then analyzed to ascertain the performance of the optic. For high performance imaging systems, such as those found in lithographic steppers used to make integrated circuits, this interferometric measurement must be made to ever increasing accuracy. The accuracy, however, is limited by how well the reference surface is characterized. Reference surfaces are typically no better than λ/50, where λ is the wavelength of visible light, and thus are the limiting factor in fabricating higher performance optical systems. Therefore the fabrication of high accuracy optical systems, such as those needed for extreme ultraviolet projection lithography which require an accuracy of λ/1000, are impossible to qualify with confidence using existing interferometry.
The problem or difficulty with interferometrically measuring a convex mirror or a negative lens is that it is necessary to have a converging spherical wavefront incident on either of these two optics in order to make a measurement with an interferometer. This problem is particularly true of the phase measuring diffraction interferometer since it produces a perfect diverging spherical wavefront.
In order to produce a converging spherical wavefront, it is necessary to introduce a positive auxiliary lens into the interferometer. This will change the perfect diverging spherical wavefront into a converging wavefront. This converging wavefront will not, in general, be a perfect spherical wavefront due to errors in the positive auxiliary lens. This will introduce an error into the measurement. In conventional interferometers this error cannot be eliminated. However, the phase shifting diffraction interferometer is unique in that it can be configured in at least two different ways, permitting exact cancellation of the error.